The present invention relates to power-assisted steering systems for wheeled vehicles, and more particularly to a changeover valve unit for controlling the supply of fluid under pressure to a hydraulic cylinder associated with the steering system.
As one of such changeover valve units as described above, there has been proposed a changeover valve unit which includes first and second shafts arranged within a valve housing for relative rotation on aligned axes, a spool valve element concentrically assembled with the first shaft to be shifted in response to relative rotation of the shafts in an axial direction to control the supply of fluid under pressure to the hydraulic cylinder, and a changeover mechanism for effecting axial movement of the spool valve element in response to relative rotation of the shafts, and in which the spool valve element is arranged to be applied therein with a back pressure from outlet ports of the valve housing in its axial movement. In such changeover valve unit, the component parts of the changeover mechanism are provided with appropriate intervening clearances for smooth operation of the valve unit. It is, however, observed that in occurrence of fluctuation of the pressure acting on the spool valve element, the provision of such intervening clearances results in undesired vibration of the spool valve element in the axial direction.
For instance, when a mechanical stopper of the system acts to restrict movement of the steering linkage caused by the operator's effort in steering the steering road wheels under arrest of the vehicle, the spool valve element is reversely moved by reaction acting thereon to receive the fluid under pressure exhaused at a high level from the hydraulic cylinder. Such reverse movement of the spool valve element will occur in the operator's effort in steering the steering road wheels in a reverse direction. As a result, the pressure acting on the spool valve element greatly fluctuates due to the fluid under pressure at the high level to cause undesired vibration of the spool valve element, resulting in occurrence of unpleasant noises and slight vibration of the steering wheel.